Filter for image sensor

Abstract: An optical filter provided on the light receiving surface of an image sensor, has a plurality of thin layers alternately laminated.

Description: Field of the Invention The present invention relates to an image sensor having a filter, and more particularly to an image sensor having a filter for converting optical signal in the not only visible but also non-visible region into electrical signal. And the invention is suitably employed for an image sensor having a filter for use in an image information processing device such as a facsimile, an image scanner, or a copying machine. Related Background Art In recent years, the uses of solid-state image pickup devices have been diversified, with new functions needed. For example, in addition to having higher image quality and color picture of the copying machine, they are required to recognize, reproduce and record an invisible image. Such image, or invisible image may be an image formed by the ink having the characteristic of absorbing the ultra-violet or infrared ray. The sensor technology for recognizing such image uses both a sensor for sensing the visible light and a sensor for sensing the invisible light. In a copying machine for forming the image by sensing only the visible light, an infrared cut-off filter has been disposed on the optical path between a solid-state image pickup device having color filters of red (R), green (G), and blue (B) provided on an IC chip of an image sensor such as a CCD and the original to enhance color separation ability. Accordingly, when reading and reproducing a printed image as the visible image, using an infrared absorbent ink, for example, one method can be conceived in which an optical system dedicated for infrared rays without infrared cut-off filter is prepared, apart from the optical path of visible light, to image the infrared rays onto the solid-state image pickup device for the detection of infrared rays. However, since the above-mentioned method is associated with a complex optical system, the present inventors have sought a new method which adopts a sensor in which a sensor for sensing the infrared rays and a sensor for sensing the RGB rays are integrated monolithically into one chip. However, this new method can not provide a conventional infrared cut-off filter in the optical path, because of its necessity of sensing the infrared rays. Accordingly, a way of disposing an invisible light cut-off filter represented by an infrared cut-off filter must be improved, because one could not possibly make effective use of the advantages of the above new method. For example, in Japanese Laid-open Patent Application No. 59-225564 or No. 62-174716, attempts have been made to integrate together a sensor for sensing the infrared rays and a sensor for sensing the RGB rays, but none of them have not referred to the specific constitution of its filter. JP-A-59230123 discloses an optical sensor comprising a photodetector, a metallic film interference filter and a sharp cut filter. The metallic film interference filter is constituted of alternate layers of a metallic film, a dielectric film, and a metallic film. The sharp cut filter is constituted so as to cut off an unnecessary transmission band. US-A-4956555 discloses a multicolor focal plane array for detecting and distinguishing between incoming electromagnetic radiation within first and second bands of wavelengths, said array comprising a substrate and a two dimensional array of detectors and first and second multilayer thin film filters. The first and second multilayer thin film filters are arranged to transmit first and second wavelength bands, respectively, so that each row of detectors detects said first and second wavelength bands, respectively. Moreover, an absorbing metal layer is provided so as to extend over the whole array. As above described, regarding what construction should be taken to use an invisible light cut-off filter as the on-chip filter, it can be said that no detailed and sufficient research or examination has been made. SUMMARY OF THE INVENTION An object of the present invention is provide an image sensor having a compact filter, which image sensor can excellently sense the optical signal in a wide wavelength range from the visible to invisible region, and perform signal processing relatively easily, the filter having the good filter characteristics for separating between the visible light and the invisible light and preventing the occurrence of a film breakage failure and exfoliation in the post-process after formation. It is a further object of the present invention to provide an image sensor comprising a first plurality of sensor arrays arranged side by side including a plurality of first photoelectric conversion elements for color separating and converting the optical signal in the visible region into the electrical signal and a second plurality of sensor arrays arranged side by side including a plurality of second photoelectric conversion elements for color separating and converting the optical signal in the invisible region into the electrical signal; a color separation filter for image sensor formed on said first photoelectric conversion elements to separate the optical signal in the visible region; an invisible-light shielding filter, formed on said first photoelectric conversion elements as a common layer on said first sensor arrays to shield the optical signal in the invisible region, said invisible-light shielding filter comprising a plurality of layers consisting of dielectric layers and metal layers. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a typical plan view of an image sensor. Fig. 2 is a typical cross-sectional view of the image sensor. Fig. 3 is a typical cross-sectional view showing a part of manufacturing flow of the image sensor. Fig. 4 is a typical cross-sectional view of a filter for image sensor according to one embodiment of the present invention. Fig. 5 is a film characteristic curve of an example 1 of the present invention. Fig. 6 is a film characteristic curve of an example 2 of the present invention. Fig. 7 is a film characteristic curve of an example 3 of the present invention. Fig. 8 is a correction film characteristic curve of an example 4 of the present invention. Fig. 9 is a composite film characteristic curve of an example 4 of the present invention. Fig. 10 is a typical cross-sectional view of a package in the example 4 of the present invention. Fig. 11 is a block diagram of an image information processing device having an image sensor according to the present invention. Fig. 12 is a spectral characteristic curve of an infrared absorption coloring matter. Fig. 13 is a view showing an image pattern. Fig. 14 is a block diagram showing the configuration of a discrimination unit of the device of Fig. 11. Fig. 15 is a diagram for explaining a discrimination method of the image pattern. Fig. 16 is a diagram showing the configuration of a hardware for carrying out a decision algorithm. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An image sensor of the present invention includes a filter which is comprised of a plurality of layers consisting of dielectric layers and metal layers and is provided on the light receiving surface of an image sensor. The dielectric film may be appropriately selected in a layer thickness of single layer from 10 to 200 nm, and the metal thin film may be appropriately selected in a layer thickness of single layer from 10 to 50 nm. The number of laminated layers is appropriately selected as necessary, depending on the required spectral characteristics and the structure of image sensor. The image sensor having a filter of the present invention may be an image sensor having a photodiode, a phototransistor or a photoconductive element as the photoelectric conversion element, the filter laminated directly or via an insulating layer on the semiconductor region which forms the light receiving face thereof. A flattened layer may be provided on the insulating layer as necessary. The image sensor is not limited to sensors capable of color separation into four components R, G, B and iR, as described below, but may be any image sensor as far as it requires removal of the invisible light, for example, a sensor for R single color, a sensor for G single color, a sensor for B single color, and a monochrome sensor. Also, it may be a sensor for complementary color such as yellow (Y), cyan (Cy) and magenta (M). One embodiment of the present invention will be now described, exemplifying the constitution of an image sensor which can sense the infrared rays in the invisible region. Fig. 1 shows a typical plan view of an image sensor to which the present invention is applied. This image sensor 1 has photoelectric conversion elements (R, G, B) for converting the optical signal in the visible region into the electrical signal, and a photoelectric conversion element (iR) for converting the optical signal in the invisible region into the electrical signal arranged side by side in four lines. The photoelectric conversion element used herein may be a photovoltaic element or a photoconductive element such as a photodiode or a phototransistor. And the photoelectric conversion element useful for the visible region has a lamination filter (iR cut-off filter) composed of a dielectric multi-layer film which can transmit the visible light and cut off the invisible light. To obtain the signal in a specific visible region, it has also a filter for transmitting selectively only the light in the specific region. That is, each of photoelectric conversion elements of R (red), G (green) and B (blue) has an inherent filter for transmitting only the light within a specific wavelength range in the visible region and a common filter for cutting off the invisible light. On the other hand, the photoelectric conversion element (iR) for converting the optical signal in the invisible region into the electrical signal may be constructed of a material having sensitivity in a wide wavelength range covering the invisible light region, in combination with a filter having selective transmittance to the light in the invisible region. As above described, by the combination of each filter, it is possible to obtain the optical signal in each specific visible region and the invisible region. Fig. 2 shows a typical cross-sectional view of an example of the image sensor. On each of photodiodes 6a to 6d made on an Si substrate 2 is formed each filter to fulfill its function. Also, CCDs 7a to 7d for transferring the optical signal are arranged adjacent photodiodes 6a to 6d, respectively. On the photodiodes 6a to 6c, filters 5c, 5d, 5e for selectively transmitting only the light in the specific wavelength range and a filter 4 for cutting off the invisible light are provided, whereby the optical signal in the visible region can be obtained by the combination of the filters 5c, 5d, 5e and filter 4. Also, on the photodiode 6d, filters 5b, 5a for selectively transmitting only the light in the wavelength range of B, R are provided, whereby the optical signal in the invisible region can be obtained. 3a, 3b is a flattened film. There is left some possibility of technical improvements in such filters for separating the visible light and the invisible light, and the manufacturing process thereof, as previously described. Fig. 3 is a typical cross-sectional view showing a forming process of a multi-layer film and its post-process. After treating a flattened film 8 (corresponding to flattened film 3a, 3b of Fig. 2), a multi-layer film 10 is provided, a pattern 11 is formed by applying a resist to remove unnecessary portion with the multi-layer film left on the visible region sensor unit, and etched by RIE. Further, the post-process includes forming an aluminum electrode pad 9, patterning of scribing portion, etching, further dicing, wirebonding, and sealing with glass 12, whereby a sensor chip is fabricated. By the way, the metal film such as Au, Ag, Al having light absorbency is widely used as a metal reflection mirror when it is thick enough to be opaque or as a heat reflection mirror which can transmit the visible light beam and reflect the infrared light beam when it is thin. On the other hand, the dielectric film applied on the metal film or both faces has the protection effect of preventing the corrosion of metal, as well as the reflection preventing effect of lowering the reflectance with interference. As a result of careful research about the filter to have less film thickness and film stress and good filter characteristics with less number of laminations, the present inventors have found that the good filter characteristics of transmitting the light in the visible region and cutting off radiation in the wide invisible region can be obtained even by the thin film, with a constitution where the high refractive index dielectric layer and the metal layer are alternately laminated to further enhance the effect due to interference between the metal layer and the dielectric layer, and thus have attained the present invention. That is, a filter for image sensor according to one embodiment of the present invention is formed on photoelectric conversion elements as a common layer on sensor arrays to shield the optical signal in the invisible region, characterized in that the filter is composed of a plurality of layers consisting of a dielectric layer 21(H) and a metal layer 22 (M), as shown in Fig. 4. Examples of the dielectric layer of the present invention may include the same material as the high refractive index film, namely, TiO 2 , ZnS, Ta 2 O 5 , Nb 2 O 3 , ZrO 2 , HfO 2 , Y 2 O 3 , CeO 2 , Al 2 O 3 and their mixtures. Also, examples of the metal layer 22 may include Au, Ag, Cu, and Al. Note that when used as the infrared cut-off filter, particularly silver or silver alloy having silver as the main component can be suitably used. Note that a filter for image sensor of the present invention is preferably manufactured by vacuum deposition, rf ion plating or sputtering from the respects of the controllability of the film thickness, the prevention of corrosion of the metal film, and the prevention of mixing of the dirt. According to the present invention, the film stress can be reduced with less number of laminations and less film thickness, but with good filter characteristics, and therefore, in forming the multi-layer film of the filter, a film breakage failure in the post-process after formation can be prevented, whereby the image sensor having good characteristics can be manufactured efficiently and stably. The above-described embodiment can offer excellent visible light transmitting characteristics as well as preventing the breakage of film by using the metal layer to reduce the number of laminated layers. The examples of the present invention will be described below using the drawings. Note that the following examples are concerned with the filters for cutting off the light mainly in the infrared region, but the present invention is not limited to such filters in the infrared wavelength region. (Example 1) After forming a flattened film of acrylic type polymer, a multi-layer film of five-layer configuration comprising alternating layers consisting of a high refractive index dielectric index layer and a metal layer was formed. The material of the high refractive index dielectric layer is titanium oxide and that of the metal layer is silver. Oxide gas was supplied into titanium oxide and argon gas supplied into silver so that the degree of vacuum at the film formation be held at 0.02 Pa, and the material of each film was evaporated in rf plasma at 100 W by resistive heating to form each film. The film thickness of each layer was set by the computer calculation, an optical film thickness monitor and a quartz film thickness monitor, with the film thickness of high refractive index layer being 30 nm per layer, and the film thickness of metal layer being 15 nm per layer, so that the total film thickness of five-layer configuration (three high refractive index dielectric layers and two metal layers) was 120 nm. It is desirable that the film thickness of high refractive index dielectric layer is selected in a range from 25 to 100 nm per layer and the film thickness of metal layer is selected in a range from 10 to 50 nm per layer. Fig. 4 shows a cross-sectional view of the film configuration and Fig. 5 shows the film characteristic. In Fig. 4, 21 is a high refractive index dielectric layer and 22 is a metal layer. After forming this multi-layer film, the post-process was conducted to fabricate plural image sensors, so that all sensors were excellently fabricated without occurrence of the film breakage. (Example 2) After forming a flattened film of acrylic type polymer, a multi-layer film of three-layer configuration comprising alternating layers consisting of a high refractive index dielectric layer, a metal layer and a high refractive index dielectric layer was formed by sputtering. The material of the high refractive index dielectric layer is titanium oxide and that of the metal layer is silver. Argon gas and nitrogen gas were supplied into silicon nitride and argon gas supplied into silver so that the degree of vacuum at the film formation be held at 0.3 Pa. The film thickness of each layer was set by the computer calculation and a quartz film thickness monitor, with the film thickness of high refractive index dielectric layer being 25 nm per layer, and the film thickness of metal layer being 25 nm per layer, so that the total film thickness of three-layer configuration was 75 nm. It is desirable that the film thickness of high refractive index dielectric layer is selected in a range from 25 to 100 nm per layer and the film thickness of metal layer is selected in a range from 10 to 50 nm per layer. Fig. 6 shows the film characteristic. After forming this multi-layer film, the post-process was conducted to fabricate plural image sensors, so that all sensors were excellently fabricated without occurrence of the film breakage. (Example 3) After forming a flattened film of acrylic type polymer, a contact layer composed of silicon oxide was provided 50 nm thick by vacuum deposition, and a multi-layer film of three-layer configuration comprising alternating layers consisting of a high refractive index dielectric layer, a metal layer and a high refractive index dielectric layer was formed thereon. The material of the high refractive index dielectric layer is titanium oxide and that of the metal layer is silver. Oxygen gas was supplied into silicon nitride so that the degree of vacuum at the film formation be held at 0.01 Pa, and the film was formed by evaporating the material of each film by resistive heating. The film thickness of each layer was set by the computer calculation, an optical film thickness monitor, and a quartz film thickness monitor, with the film thickness of high refractive index dielectric layer being 50 nm per layer, and the film thickness of metal layer being 25 nm per layer, so that the total film thickness of three-layer configuration (the first layer constituting a dielectric layer lamination film) was 125 nm. It is desirable that the film thickness of high refractive index dielectric layer is selected in a range from 25 to 100 nm per layer and the film thickness of metal layer is selected in a range from 10 to 50 nm per layer. Fig. 7 shows the film characteristic. After forming this multi-layer film, the post-process was conducted to fabricate plural image sensors, so that all sensors were excellently fabricated without occurrence of the film breakage. (Example 4) A multi-layer film of five-layer film configuration similar to that of example 1 was formed, with a filter film for correcting the light sensitivity in the visible region and the invisible region provided on the outside surface of sensor, and then an image sensor was fabricated by performing the post-process. Fig. 8 shows a correction film characteristic, Fig. 9 shows a composite film characteristic, and Fig. 10 shows a typical cross-sectional view of a package. In Fig. 10, 31 is a sealing glass, 32 is a correction film, and 33 is a sensor. It does not matter that a filter for correction may be installed in the optical path leading to the external optical system as conventionally. As above described, with the examples of the present invention, it is possible to form a filter composed of a multi-layer film directly on the image sensor, serving for the diversification of the function and usage of the image sensor. In particular, it is possible to obtain the filter characteristic with small film thickness on the flattened film, and thus to provide the image sensor with excellent characteristics and its manufacturing method by preventing the film breakage or film exfoliation in the post-process. (Image Information Processing Device) An image information processing device using an image sensor of the present invention will be described below. Fig. 11 shows a block diagram, wherein the signal from a solid-state image pickup device (sensor) 1 of each example as described is entered into an image processing unit 1003 and a discrimination unit 1001 as decision means. And the information thereof is reproduced and recorded by a recording unit 1005 which is driven by a recording control 1004. The sensor 1 reads the original at a pixel density of 400 dpi by separating substantially the same point of the original into R (red), G (green), B (blue) components, and additionally an infrared component having sensitivity near roughly 1000 nm. The sensor output is subjected to so-called shading correction, using a white plate and an infrared reference plate, and entered each as 8-bit image signal into the discrimination unit 1001 and the image processing unit 1003. The image processing unit 1003 performs various processings including variable magnification masking, OCR and so on which are typically performed in the color copying machine to produce four color signals of C, M, Y and K which are recording signals. On the other hand, the discrimination unit 1001 performs the detection of specific patterns in the original which is characteristic of the present invention, with its result being output to the recording control 4, where the record signal is processed as necessary, such as painting with a specific color, and recorded on the recording sheet by the recording unit 1005, or the recording is stopped to disenable faithful image reproduction. The image pattern to be detected in the present invention will be outlined below using Figs. 12 and 13. Fig. 12 shows a spectral characteristic of a transparent coloring matter which is substantially transmissive in the visible region and can absorb infrared rays near 800 nm, typical example of which is SIR-159 made by Mitsui Toatsu Chemicals, Inc. Fig. 13 is a pattern example made by using the transparent ink composed of the above transparent infrared absorbent coloring matter. That is, a smaller pattern b of a square having a side length of about 120 pm is printed by using the above transparent ink on a triangular pattern recorded with an ink a for reflecting a particular or infrared ray. Since the same pattern is substantially of the same color in the visible region, as shown in the figure, the pattern b is indiscernible to the human's eyes, but is possible to detect in the infrared region. Note that though a pattern having a side length of about 120 µm was exemplified for the later description, but if this area b is read at 400 dpi, the area b can be read at 400 dpi to be as large as about 4 pixels as shown. Note that the forming method of the pattern is not limited to this example. The details of the discrimination unit 1001 of Fig. 11 will be further described using Fig. 14. In Fig. 14, 10-1 to 10-4 are image data delay portions constituted of FiFo, each delaying image data of 32 bits (8 bits x 4 components) for one line. As input image signal is first delayed and held 2 pixels by flip-flops 11-1, 11-2 to have A pixel data, delayed 2 lines by memories 10-1, 10-2 to have C pixel data, further delayed 2 pixels by FF 11-3, 11-4 to have pixel data of notice X, and similarly delayed 2 pixels by FF 11-5, 11-6 to have B pixel data, and similarly D pixel data is obtained, which pixel data are input into the discrimination unit 12 simultaneously. Herein, the positional relation of A, B, C and D in the neighborhood of the pixel position of notice X is represented in Fig. 15. That is, if the pixel of notice X reads the ink of portion b in Fig. 13, it follows that A, B, C and D read the image of a pattern located around it. (Decision Algorithm) It is supposed that R component constituting a pixel signal of A is A R , G component is A G , B component is A B , infrared component is A IR , and each component of R, G, B, IR constituting each pixel signal B, C, D is likewise defined. And the average values Y R , Y G , Y B , Y IR of the same color components are obtained by the following expressions: Y R = 1/4(A R + B R + C R + D R ) Y G = 1/4(A G + B G + C G + D G ) Y B = 1/4(A B + B B + C B + D B ) Y IR = 1/4(A IR + B IR + C IR + D IR ) The decision of the pattern of object follows the difference between the average values obtained by the above expressions and the pixel of notice X. That is, the presence of a pattern is decided, if ΔR < K ΔG < K (K, L constant) ΔB < K ΔIR > L where ΔR = |Y R - X R |, ΔG = |Y G - X G |, ΔB = |Y B - X B |, ΔIR = Y IR - X IR . That is, it can be decided that the pixel of notice has smaller differences in color tint from its surrounding pixels in the visible region, and a difference of a constant L or greater in the infrared characteristic. Fig. 16 is a hardware example in which the above decision algorithm is implemented. An adder 121 adds simply a color component of each of four pixels and outputs upper 8 bits thereof to obtain Y R , Y G , Y B and Y IR , respectively. A subtracter 122 takes the difference of each component from the pixel of notice signal, and for three components of R, G and B, each absolute value is compared with a constant K as the reference signal by a comparator 126. Each output from the comparator as above is entered into an AND gate 127, in which a pattern is decided if "1" appears at its output terminal.

Claims: An image sensor comprising: a first plurality of sensor arrays (7a, 7b, 7c) arranged side by side including a plurality of first photoelectric conversion elements (6a, 6b, 6c) for color separating and converting an optical signal in the visible region into an electrical signal; a second sensor array (7d) including a plurality of second photoelectric conversion elements (6d) and converting the optical signal in the invisible region into an electrical signal wherein said first and second sensor arrays are arranged side by side; a color separation filter (5c, 5d, 5e) formed over said first photoelectric conversion elements to separate the optical signal in the visible region; and an invisible-light shielding filter (4), formed over said first photoelectric conversion elements (6a, 6b, 6c) as a common layer on said first sensor arrays, to shield the optical signal in the invisible region, said invisible-light shielding filter (4) comprising a plurality of layers consisting of dielectric layers (21) and metal layers (22). An image sensor according to claim 1, wherein said invisible-light shielding filter (4) comprises alternating dielectric layers (21) and metal layers (22), the dielectric layers having a high refractive index. An image sensor according to claim 1, wherein said metal layers (22) are composed of a material selected from the group consisting of silver or a silver alloy having silver as its main component. An image sensor according to claim 2, wherein said dielectric layer (21) is composed of a material selected from the group consisting of aluminum oxide, zirconium oxide, titanium oxide, hafnium oxide, yttrium oxide, tantalum oxide, niobium oxide, silicon oxide, zinc oxide or a mixture thereof. An image sensor according to claim 1, wherein said color separating filter (5c, 5d, 5e) includes a red light transmitting filter, a blue light transmitting filter and a green light transmitting filter. An image sensor according to claim 1, wherein said invisible light shielding filter (4) is arranged over said color separation filter. An image sensor according to claim 1, wherein said image sensor is a complementary color image sensor. An image sensor according to claim 1, wherein each of said first photoelectric conversion elements (6a, 6b, 6c, 6d) includes a photodiode. An image sensor according to any of claims 1, 5, 6, 7 or 8, wherein said invisible-light shielding filter (4) is an infrared ray cut filter. An image sensor according to claim 6, wherein between said color separation filter (5c, 5d, 5e) and said invisible-light shielding filter (4), a flattened layer (3b) is provided. An image sensor according to claim 6, wherein said invisible-light shielding filter (4) is provided on said color separation filter. An image information processing device comprising: an image sensor according to any of claims 1 to 4; image forming means for forming an image based on an optical signal in the visible region; and control means for controlling operation of said image forming means in response to an optical signal in the invisible region.

Inventors: FUKASAWA MOTOMU, SAKAMOTO JUNICHI, SAWAMURA MITSUHARU, TSUKAMOTO SEITOKU, FUKASAWA, MOTOMU, SAKAMOTO, JUNICHI, SAWAMURA, MITSUHARU, TSUKAMOTO, SEITOKU, FUKASAWA, MOTOMU, C/O CANON KABUSHIKI KAISHA, SAKAMOTO, JUNICHI, C/O CANON KABUSHIKI KAISHA, SAWAMURA, MITSUHARU, C/O CANON KABUSHIKI KAISHA, TSUKAMOTO, SEITOKU, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1994
Main classification: H01L31
IPC codes: H01L31